NEET MDS Lessons
Physiology
Structure and function of skeletal muscle.
Skeletal muscles have a belly which contains the cells and which attaches by means of tendons or aponeuroses to a bone or other tissue. An aponeurosis is a broad, flat, tendinous attachment, usually along the edge of a muscle. A muscle attaches to an origin and an insertion. The origin is the more fixed attachment, the insertion is the more movable attachment. A muscle acts to shorten, pulling the insertion toward the origin. A muscle can only pull, it cannot push.
Muscles usually come in pairs of antagonistic muscles. The muscle performing the prime movement is the agonist, the opposite acting muscle is the antagonist. When the movement reverses, the names reverse. For example, in flexing the elbow the biceps brachii is the agonist, the triceps brachii is the antagonist. When the movement changes to extension of the elbow, the triceps becomes the agonist and the biceps the antagonist. An antagonist is never totally relaxed. Its function is to provide control and damping of movement by maintaining tone against the agonist. This is called eccentric movement.
Muscles can also act as synergists, working together to perform a movement. This movement can be different from that performed when the muscles work independently. For example, the sternocleidomastoid muscles each rotate the head in a different direction. But as synergists they flex the neck.
Fixators act to keep a part from moving. For example fixators act as postural muscles to keep the spine erect and the leg and vertebral column extended when standing. Fixators such as the rhomboids and levator scapulae keep the scapula from moving during actions such as lifting with the arms.
Maintenance of Homeostasis
The kidneys maintain the homeostasis of several important internal conditions by controlling the excretion of substances out of the body.
Ions. The kidney can control the excretion of potassium, sodium, calcium, magnesium, phosphate, and chloride ions into urine. In cases where these ions reach a higher than normal concentration, the kidneys can increase their excretion out of the body to return them to a normal level. Conversely, the kidneys can conserve these ions when they are present in lower than normal levels by allowing the ions to be reabsorbed into the blood during filtration. (See more about ions.)
pH. The kidneys monitor and regulate the levels of hydrogen ions (H+) and bicarbonate ions in the blood to control blood pH. H+ ions are produced as a natural byproduct of the metabolism of dietary proteins and accumulate in the blood over time. The kidneys excrete excess H+ ions into urine for elimination from the body. The kidneys also conserve bicarbonate ions, which act as important pH buffers in the blood.
Osmolarity. The cells of the body need to grow in an isotonic environment in order to maintain their fluid and electrolyte balance. The kidneys maintain the body’s osmotic balance by controlling the amount of water that is filtered out of the blood and excreted into urine. When a person consumes a large amount of water, the kidneys reduce their reabsorption of water to allow the excess water to be excreted in urine. This results in the production of dilute, watery urine. In the case of the body being dehydrated, the kidneys reabsorb as much water as possible back into the blood to produce highly concentrated urine full of excreted ions and wastes. The changes in excretion of water are controlled by antidiuretic hormone (ADH). ADH is produced in the hypothalamus and released by the posterior pituitary gland to help the body retain water.
Blood Pressure. The kidneys monitor the body’s blood pressure to help maintain homeostasis. When blood pressure is elevated, the kidneys can help to reduce blood pressure by reducing the volume of blood in the body. The kidneys are able to reduce blood volume by reducing the reabsorption of water into the blood and producing watery, dilute urine. When blood pressure becomes too low, the kidneys can produce the enzyme renin to constrict blood vessels and produce concentrated urine, which allows more water to remain in the blood.
Neural Substrates of Breathing
A. Medulla Respiratory Centers
Inspiratory Center (Dorsal Resp Group - rhythmic breathing) → phrenic nerve→ intercostal nerves→ diaphragm + external intercostals
Expiratory Center (Ventral Resp Group - forced expiration) → phrenic nerve → intercostal nerves → internal intercostals + abdominals (expiration)
1. eupnea - normal resting breath rate (12/minute)
2. drug overdose - causes suppression of Inspiratory Center
B. Pons Respiratory Centers
1. pneumotaxic center - slightly inhibits medulla, causes shorter, shallower, quicker breaths
2. apneustic center - stimulates the medulla, causes longer, deeper, slower breaths
C. Control of Breathing Rate & Depth
1. breathing rate - stimulation/inhibition of medulla
2. breathing depth - activation of inspiration muscles
3. Hering-Breuer Reflex - stretch of visceral pleura that lungs have expanded (vagal nerve)
D. Hypothalamic Control - emotion + pain to the medulla
E. Cortex Controls (Voluntary Breathing) - can override medulla as during singing and talking
Functional Divisions of the Nervous System:
1) The Voluntary Nervous System - (ie. somatic division) control of willful control of effectors (skeletal muscles) and conscious perception. Mediates voluntary reflexes.
2) The Autonomic Nervous System - control of autonomic effectors - smooth muscles, cardiac muscle, glands. Responsible for "visceral" reflexes
Nucleic Acids:
- Two major types: DNA
- RNA (including mRNA, tRNA, & rRNA)
- Both types have code which specifies the sequence of amino acids in proteins
- DNA = archival copy of genetic code, kept in nucleus, protected
- RNA = working copy of code, used to translate a specific gene into a protein, goes into cytoplasm & to ribosomes, rapidly broken down
- Nucleic acids are made of 5 nucleotide bases, sugars and phosphate groups
- The bases make up the genetic code ; the phosphate and sugar make up the backbone
- RNA is a molecule with a single strand
- DNA is a double strand (a double helix) held together by hydrogen bonds between the bases
- A = T; C= G because:
- A must always hydrogen bond to T
- A = T; C= G because:
C must always hydrogen bond to G
Bile contains:
- bile acids. These amphiphilic steroids emulsify ingested fat. The hydrophobic portion of the steroid dissolves in the fat while the negatively-charged side chain interacts with water molecules. The mutual repulsion of these negatively-charged droplets keeps them from coalescing. Thus large globules of fat (liquid at body temperature) are emulsified into tiny droplets (about 1 µm in diameter) that can be more easily digested and absorbed.
- bile pigments. These are the products of the breakdown of hemoglobin removed by the liver from old red blood cells. The brownish color of the bile pigments imparts the characteristic brown color of the feces.
The small intestine
Digestion within the small intestine produces a mixture of disaccharides, peptides, fatty acids, and monoglycerides. The final digestion and absorption of these substances occurs in the villi, which line the inner surface of the small intestine.
This scanning electron micrograph (courtesy of Keith R. Porter) shows the villi carpeting the inner surface of the small intestine.
The crypts at the base of the villi contain stem cells that continuously divide by mitosis producing
- more stem cells
- cells that migrate up the surface of the villus while differentiating into
- columnar epithelial cells (the majority). They are responsible for digestion and absorption.
- goblet cells, which secrete mucus;
- endocrine cells, which secrete a variety of hormones;
- Paneth cells, which secrete antimicrobial peptides that sterilize the contents of the intestine.
All of these cells replace older cells that continuously die by apoptosis.
The villi increase the surface area of the small intestine to many times what it would be if it were simply a tube with smooth walls. In addition, the apical (exposed) surface of the epithelial cells of each villus is covered with microvilli (also known as a "brush border"). Thanks largely to these, the total surface area of the intestine is almost 200 square meters, about the size of the singles area of a tennis court and some 100 times the surface area of the exterior of the body.
Incorporated in the plasma membrane of the microvilli are a number of enzymes that complete digestion:
- aminopeptidases attack the amino terminal (N-terminal) of peptides producing amino acids.
- disaccharidasesThese enzymes convert disaccharides into their monosaccharide subunits.
- maltase hydrolyzes maltose into glucose.
- sucrase hydrolyzes sucrose (common table sugar) into glucose and fructose.
- lactase hydrolyzes lactose (milk sugar) into glucose and galactose.
Fructose simply diffuses into the villi, but both glucose and galactose are absorbed by active transport.
- fatty acids and monoglycerides. These become resynthesized into fats as they enter the cells of the villus. The resulting small droplets of fat are then discharged by exocytosis into the lymph vessels, called lacteals, draining the villi.